The mechanism which couples depolarization of the cell membrane to the evaluation of intracellular Ca2+ which results in contraction continues to be one of the most intriguing features of muscle physiology. In all muscle types this mechanism is subject to modulation, the classical example being the regulation of myocardial contraction by neurotransmitters and drugs. The broad, long-term objectives of this research are to gain an understanding of excitation-contraction coupling in muscles that require extracellular Ca1+ for contractile activation, and its modulation by neurotransmitters and drugs, using the ventroabdominal flexor muscle of the crustacean Atya lanipes as a model. The specific aims of the project are to: 1) use immunohistochemical methods to determine the presence of proctolin and octopamine in neurons in the ventral nerve cord or nerve endings on muscle fibers; 2) determine if these substances enhance Ca2+ currents in muscle fibers and begin to explore the mechanism by which Ca+ currents are enhanced; 3) determine the threshold membrane potential for contractile activation, and assess the effects of extracellular Ca2+, tetracaine, proctolin and octopamine on contractile activation; 4) characterize nonlinear charge movement in Atya skeletal muscle and determine whether proctolin and octopamine alter charge movement; 5) determine, detubulating the fibers with the glycerol method, the localization of the Ca2+ channels. The results of these experiments may provide important insights into the mechanisms that link membrane potential changes to the generation of tension in this and other Ca2+- dependent muscles.